Raman spectroscopy is an analytic instrumentation methodology useful in ascertaining and verifying the molecular structures of materials. Raman spectroscopy relies on inelastic scattering, or Raman scattering, of monochromatic light incident on a sample. Raman scattering results in an energy shift in a portion of the photons scattered by a sample. From the shifted energy of the Raman scattered photons, vibrational modes characteristic to a specific molecular structure can be ascertained. This is the basis of using Roman spectroscopy to ascertain the molecular makeup of a sample. In addition, by analytically assessing the relative intensity of Raman scattered photons, the purity of a sample can be determined.
Typically, a sample is illuminated with a laser beam. Light reflected/scattered by the sample is collected by lenses and analyzed. Most of the reflected light is at wavelengths close to the laser line, which are due to elastic Rayleigh scattering. A small fraction of the collected light consists of Raman scattered photons. These selected bands of the collected light are directed onto a detector for quantitative analysis.
The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud and the bonds of that molecule. For the spontaneous Raman effect, which is a form of light scattering, a photon excites the molecule from its ground state to a virtual energy state. The energy state is referred to as virtual since it is temporary, and not a discrete (real) energy state. When the molecule relaxes, it emits a photon and it returns to a different rotational or vibrational state. The difference in energy between the original state and this new state leads to a shift in the emitted photon's frequency away from the excitation wavelength.
If the final vibrational state of the molecule is more energetic than the initial state, then the emitted photon will be shifted to a lower frequency in order for the total energy of the system to remain balanced. This shift in frequency is known as a Stokes shift. If the final vibrational state is less energetic than the initial state, then the emitted photon will be shifted to a higher frequency, which is known as an anti-Stokes shift. An optical signal containing Stokes or anti-Stokes shifted photons is referred to herein as a Stokes or anti-Stokes scatter, respectively. Both are forms of Raman scattering. Raman scattering is an example of inelastic scattering because of the energy transfer between the photons and the molecules during their interaction.
The pattern of shifted frequencies is determined by the rotational and vibrational states of the sample, which are characteristic of the molecules. The chemical makeup of a sample may thus be determined by quantitative analysis of the Raman scattering.
Conventional Raman spectroscopy relies on a complex, sensitive, carefully calibrated optical system comprising a laser providing a source beam; an array of photodetectors for detecting Stokes and anti-Stokes shifted photons; optics, including lenses, mirrors, and optical filters; and data processing systems. Conventional Raman spectroscopy systems are maintained in a controlled environment, such as a laboratory.
A standing challenge in Raman spectroscopy is achieving a high signal-to-noise ratio in the detection and analysis of Raman scattered photons. The vast majority of photonic return from illuminating a sample with a laser is unshifted in frequency from the incident light. This unshifted return may comprise photons resulting from specular reflection, diffuse reflection, Mie scatter, and/or Rayleigh scatter—collectively referred to herein as “unshifted return.” The unshifted return typically swamps the Stokes and anti-Stokes components, which are at wavelengths that are shifted in frequency from the incident laser. A need exists in the art for a reliable way to detect, amplify, and process the Raman scattered, or shifted, photonic energy returned from illuminating a sample, in the presence of unshifted return.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.